Department of Chemistry

Abstract

[Cu(TpPh)(L)](ClO4) (1-4), where TpPh is anionic hydrotris(3-phenylpyrazolyl)borate and L is N,N-donor heterocyclic base, viz. 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,2-f:2',3'-h]quinoxaline (dpq, 3), and dipyrido[3,2-a;2',3'-c]phenazine (dppz, 4), were prepd. from a reaction of Cu(II) acetate hydrate dimer with KTpPh and L in CH2Cl2 and isolated as perchlorate salts. The complexes were characterized by anal., structural, and spectral methods. The crystal structures of complexes 1-4 show discrete cationic complexes having the metal, TpPh, and L in a 1:1:1 ratio and a noncoordinating perchlorate anion. The complexes have a square-pyramidal 4 + 1 coordination geometry in which two nitrogens of L and two nitrogens of the TpPh ligand occupy the basal plane and one N of TpPh binds at the axial site. Complexes 3 and 4 display distortion from the square-pyramidal geometry. The Cu-N distances for the equatorial and axial positions are ∼2.0 and 2.2 Å, resp. The Ph groups of TpPh form a bowl-shaped structure that encloses the {CuL} moiety. The steric encumbrance is greater for the bpy and phen ligands compared to that for dpq and dppz. The 1-electron paramagnetic complexes (μ ≈ 1.8 μB) exhibit axial EPR spectra in CH2Cl2 glass at 77 K giving g.dblvert. and g.perp. values of ∼2.18 (A.dblvert. = 128 G) and ∼2.07. The data suggest a {dx2-y2}1 ground state. The complexes are redox-active and display a quasi-reversible cyclic voltammetric response for the Cu(II)/Cu(I) couple near 0.0 V vs. SCE with an ipc/ipa ratio of unity in CH2Cl2 or DMF-0.1 M TBAP. The E1/2 values of the couple vary in the order 4 > 3 > 2 > 1. A profound effect of steric encumbrance caused by the TpPh ligand is obsd. in the reactivity of 1-4 with the calf thymus (CT) and supercoiled (SC) DNA. Complexes 2-4 show similar binding to CT DNA. The propensity for the SC DNA cleavage varies as 4 > 3 > 2. The bpy complex does not show any significant binding or cleavage of DNA. Mechanistic studies using distamycin reveal minor groove binding for 2 and 3 and a major groove binding for 4. The scission reactions that are inhibited by hydroxyl radical scavenger DMSO probably proceed through sugar hydrogen abstraction pathways. [on SciFinder(R)]